ABSTRACT Undernutrition is a significant health risk for children under the age of five, particularly in low and middle-income countries (LMICs). The microbiome of breast milk has recently been suggested to contribute to colonization of a maturing infant gut, which is subsequently linked to healthy growth and nutritional status of the child. Helminth infections, including schistosomiasis and hookworm infections, have been associated with microbial translocation across the affected individual's gut, causing dysbiosis detectable in circulation. Preliminary data from our group show that endotoxin levels in the circulation of pregnant women are higher in cases of either schistosomiasis and/or hookworm and total protein and lactoferrin levels in breast milk are lower. This proposal is the first to examine the impact of schistosomiasis and/or hookworm on the microbiome of breast milk, which can be a critical player in colonization of the infant gut and long-term growth trajectory of the child. We will leverage samples from an ongoing NIAID funded K01 study evaluating the impact of maternal schistosomiasis on metabolic programming of the infant. Specific Aim 1 will determine if the microbiome of breast milk is altered in cases of maternal helminth infection. We will use 16S amplicon and whole genome shotgun sequencing to identify the unique microbial signature in breast milk samples from one month of infant age as well as maternal stool and skin samples. Genera dominant in either milk or gut microbiome will be examined to test the hypothesis that gut microbes are inappropriately present in breast milk of helminth infected women. Specific Aim 2 will examine the relationship between breast milk microbiome and nutritional quality of milk. We will measure fat, protein, alpha-lactalbumin, lactose, immunoglobulin A levels and somatic cell counts in breast milk samples collected at one, three, six and twelve months of infant age. This work will be the first investigation regarding the association between maternal helminth infection and the microbiome of breast milk, as well as impacts on nutritional quality of breast milk. The project has important implications for long-term infant growth.